Oxidation-resistant turbine blades



A ril 14, 1964 D. K. HANINK ETAL 3,129,059 OXIDATION-RESISTANT TURBINEBLADES Original Filed Oct. 11, 1956 s Sheets-Sheet 1 WIDE AFFECTED ION!E M/CMSTITl/ENT "(K/(MESS OF L UM.

NICKEL ALLOY LAYER AL PM ILLOY LAYER INVENWPS 8485 Menu AT TOPNE Y Apnl14, 1964 D. K. HANINK ETAL 3,129,069

OXIDATION-RESISTANT TURBINE BLADES orlginal Filed Oct. 11, 1956 3Sheets-Sheet 2 AL. RICH ALL 0) LAYEI? BASE ME TA.

AL RICH ALLOY LAYER 84 SE METAL 01/05 DEPTH 0F ALLOY pgpry DEPLETION AND-1 MICROSTRIATMQE CHMVE BASE METAL INVENTORS AT TGPNE Y 97 gypz w l w 6E T a ,4, k a

k 2 &

April 14, 1964 D. K. HANINK ETAL 3,129,059

OXIDATION-RESISTANT TURBINE BLADES Original Filed Oct. 11, 1956 3Sheets-Sheet 3 Suki-ACE OXIDE OEPTH OF ALL OY PENETRATION BASE METAL 400600 800 I000 I200 I400 I600 W0 0 200 {@299 77ME /-/auRs 0 I0 I0 7D PIPTIME -$ECOND$ 0.6 0.8 1.0 /.2 14 6 /.a 20 2.2 2.4 z THICKNESS OFALLOYINM/l. fgz'gfl A TTOPNE Y United States Patent Office 3,129,069 PatentedApr. 14, 1964 3,129,069 OXIDATION-RESISTANT TURBINE BLADES Dean K.Hanink, Indianapolis, Ind., and Erwin R. Price,

Detroit, Mich, assignors to General Motors Corporation, Detroit, Mich acorporation of Delaware Original application Oct. 11, 1956, Ser. No.615,417, now Patent No. 3,000,755, dated Sept. 19, 1961. Divided andthis application Dec. 17, 1959, Ser. No. 860,147 6 Claims. (Cl.29-183.5)

This invention relates to oxidation-resistant turbine buckets and nozzleguide vanes for gas turbine engines. More particularly, the invention isconcerned with nickel base alloy and cobalt base alloy turbine bucketshaving surfaces provided with a protective layer of an alloy of the basemetal with aluminum. The present application is a division of ourco-pending patent application Serial No. 615,417, now Patent No.3,000,755, which was filed on October 11, 1956 as a continuation-in-partof patent application Serial No. 506,201, filed on May 5, 1955, and nowabandoned.

High-temperature alloy components, such as turbine buckets and nozzleguide vanes, of gas turbine engines are subjected to extended periods ofservice at elevated temperatures under variable stress conditions. Whensuch components are formed of certain high-temperature nickel basealloys and cobalt base alloys, they possess excellent strength undermost high-temperature service conditions. However, durability of theseturbine buckets and nozzle guide vanes is materially reduced because ofinadequate resistance to oxide penetration. As a result, maximum usecannot be made of such high strength alloys, despite their outstandingcapacity to sustain high stresses at elevated temperatures undernonoxidizing conditions.

Moreover, as the operating temperatures of gas turbines of the type usedin turbojet and turboprop engines are increased, the problem ofoxidation of gas turbine buckets becomes more acute. Today turbineengine manufacturers are designing gas turbine engines having operatingtemperatures as high as approximately 1900 F. At this very hightemperature, oxidation of the nickel base and cobalt base alloys ofwhich turbine buckets and nozzle guide vanes are conventionally formedseriously restricts the operating life of gas turbine engines.Nevertheless, such high-operating temperatures are desirable in order toobtain maximum thrust from the engines.

A principal object of the present invention, therefore, is to provide aturbine blade which will withstand higher operating temperatures thanthose presently being used. A further object of the invention is toeliminate oxidation of the base metal constituents, particularly alonggrain boundaries or preferred crystallographic planes, of nickel basealloy and cobalt base alloy gas turbine buckets and nozzle guide vanes.A still further object of the subject invention is to provide suchturbine engine components with this protection without adverselyaffecting the loadcarrying ability of the base alloy.

Another object of the invention is to provide nickel base alloy andcobalt base alloy turbine blades with a protective surface layer whichpossesses suflicient duc tility to yield with the base metal when thelatter is expanding, contracting, or stretching due to creep elongationunder stress at elevated temperature. A further object of this inventionis to provide a turbine bucket formed of nickel base alloy or cobaltbase alloy and having, at its exposed surfaces, an integral layer ofaluminum with the base material which affords effective protectionagainst high-temperature oxidation. Although in the case of a nickelbase alloy this layer is referred to herein as an aluminum-nickel layeror an aluminum-nickel alloy layer, it will be understood that the layerconsists essentially of aluminum in combination with all the variousconstituents in the nickel base alloy of which the blade is formed.Analogous terminology is similarly employed to describe the protectivesurface layer formed on cobalt base alloys. Examples of suitable nickelbase alloys and cobalt base alloys are hereinafter set forth.

A still further object of the present invention is to provide a processfor forming a thin stable layer of the aforementioned type ofaluminum-nickel alloy or aluminumcobalt alloy at the surfaces of turbinebuckets and nozzle diaphragms formed of nickel base alloys or cobaltbase alloys, as the case may be, in such a manner that this layer willnot flake or spall during the normal operating life of the engine inwhich these blades are installed.

The above and other objects are attained in accordance with thisinvention by providing a thin aluminum coating on surfaces of nickelbase alloy and cobalt base alloy turbine buckets and nozzle guide vanes.On diffusion heat treatment, this aluminum coating further combines withthe base alloy to form a layer of aluminum-nickel alloy oraluminum-cobalt alloy. Of course, some of the aluminum usually remainson the surface of this layer in the form of a thin overlay of aluminumoxides. The diffused aluminum-nickel alloy and aluminum-cobalt alloylayers are tough, resilient and possess good ductility. Under operatingconditions, these alloy layers prevent oxide penetration of the basematerial, thereby improving the durability of the turbine buckets underhigh-temperature operating conditions. This protection is provided understatic stress conditions, non-load conditions, impact load conditions,or hot working or forging conditions.

While the invention is hereinafter specifically described principally inconnection with nickel base alloy turbine blades, it Will be understoodthat the process set forth is also applicable to cobalt base parts.

Other objects and advantages of the present invention will more fullyappear from the following detailed description of preferred embodimentsof the invention, reference being made to the accompanying drawings, inwhich:

FIGURE 1 is an elevational view, with parts broken away and in section,of a turbine bucket formed of a nickel base alloy provided with analuminum-nickel surface layer in accordance with the invention;

FIGURE 2 is a photomicrographic view of a high temperaturecreep-resistant nickel base alloy, showing the metailographic structureof the alloy near its working surface after it had been exposed tocyclic heating at elevated temperatures;

FIGURE 3 is a photomicrographic view of the nickel base alloy shown inFIGURE 2 provided with a surface layer of aluminum-nickel alloy inaccordance with this invention;

FIGURE 4 is a photomicrographic view of the alloy shown in FIGURE 3having an outer aluminum-nickel layer in accordance with the invention,showing its metallographic structure after exposure to cyclic heating atelevated temperatures;

FIGURE 5 is a photomicrographic view of the coated alloy shown in FIGURE3 which had been heat treated prior to testing, showing itsmetallographic structure after exposure to cyclic heating at elevatedtemperatures;

FIGURE 6 is a photomicrographic view of the coated alloy shown in FIGURE3 which had been treated by a pickling process prior to heat treatmentand testing, showing its metallographic structure after exposure tocyclic heat at elevated temperature;

FIGURE 7 is a photomicrographic view of the uncoated nickel base alloyshown in FIGURE 2 after being subjected to a stress-rupture test atelevated temperature;

FIGURE 8 is a photomicrographic view of the coated nickel base alloyshown in FIGURE after being subjected to a similar high-temperaturestress-rupture test;

FIGURE 9 is a graph comparing time-elongation characteristics atelevated temperature of an uncoated nickel base alloy stress-rupture barwith a. stress-rupture bar of the same alloy provided with a surfacelayer of aluminumnickel alloy in accordance with the invention;

FIGURE 10 is a graph showing the effect of dipping temperature and timeon the thickness of the diffused aluminum-nickel alloy layer; and

FIGURE 11 is a graph showing, after heat treatment, the effect ofaluminum coating bath temperature and composition on the thickness ofthe diffused aluminumnickel alloy layer.

Tht layer of aluminum-nickel alloy or aluminum-cobalt alloy, as the casemay be, may be provided at the surfaces of the nickel base or cobaltbase turbine bucket or nozzle diaphragm in any desired manner. Thepreferred method is to apply molten aluminum or aluminum base alloy tothe turbine blade under conditions such that the aluminum will form analloy with the nickel or cobalt and result in the desired alloy layerthickness. Best results are obtained when the aluminum or aluminum basealloy is applied by any of the procedures described in United StatesPatent No. 2,569,097 Grange et al., owned by the assignee of the presentinvention.

An especially advantageous method comprises preheating the turbine bladeto a temperature between approximately l280 F. and 1400 F. in a fusedsalt bath consisting essentially of 37% to 57% KCl, 25% to 45% NaCl, 8%to 20% Na AlF and 0.5% to 12% AlF The heated turbine blade is thereafterimmersed for a short time in a molten bath of aluminum or aluminum basealloy at a temperature of about 1250 F. to 1325 F. Ordinarily, theturbine blade being coated is retained in the molten aluminum oraluminum base alloy not more than approximately 10 seconds, a periodbetween 5 and 10 seconds being preferred at present.

Subsequently, the turbine blade is removed from the aluminum bath andrinsed for a short period of time not in excess of approximately 15seconds in the fluxing salt. Best results are obtained if the total diptime in the aluminum coating bath and the subsequent salt bath isbetween 10 and seconds. The excess coating material, which is still in asemi-molten or mushy condition, is then removed by rapidly vibrating theturbine blade, preferably while it is still immersed in the salt.Alternatively, an air blast may be employed to remove the surpluscoating material. As thus treated, the turbine blade is provided with anextremely thin and uniform coating of aluminum bonded to the nickel basemetal or cobalt base metal by an intermediate extremely thin and uniformlayer of an alloy of aluminum-nickel or aluminum-cobalt.

The surfaces of the turbine blade to be coated are preferably cleanedprior to the aluminum coating and alloying operation. One satisfactorymethod is to clean the blade in a molten electrolytic caustic salt (suchas the commercially available product called Kolene) at a temperature ofabout 900 F. The blade then may be washed in water and thereafterpreferably further cleaned by acid pickling. A suitable acid picklingbath is an aqueous solution containing about 2% hydrofluoric acid, 7%sulfuric acid and 10% nitric acid. In some instances, the turbine blademay require only a simple degreasing treatment in a chlorinated solventprior to the aluminum coating and alloying operation. Mechanicalcleaning methods, such as grit blasting, sand blasting, hydroblasting,etc., may be employed in some cases to supplement the chemicaltreatment.

The steps of degreasing and pickling the turbine blade are not essentialto the process, however, as heating in the fused salt prior to immersionin the aluminum or aluminum base alloy bath will provide the turbineblade 4 with clean surfaces unless it has been exceptionallycontaminated initially.

After the nickel base or cobalt base turbine blade has been cleaned, anyportions thereof which are not to be coated, such as the attachingportions at the base of the blade, may be treated with a suitablestop-off coating to prevent the aluminum from bonding to or alloyingwith the base metal at such surfaces. A suitable stop-off material forthis purpose is a sodium silicate solution, such as an aqueous solutioncontaining 20% to 50% sodium silicate.

It will be understood that variations in the aluminum coating methodhereinbefore described may be made without departing from the scope ofthe present invention. For example, the aluminum may be applied to theturbine blade in the form of a paste or paint as described in UnitedStates Patent No. 2,885,304 Thomson et al. An example of the aluminumpaste or paint which may be used is a mixture of aluminum powder withsuitable amounts of a vehicle, such as low ash content lacquer or resinsolution, liquid lucite or a water solution of salt flux. Thus, aluminumpowder may be mixed with a suitable resinous carrier, such as vinyl oracrylic resins in appropriate organic solvents, and applied by brushing,spraying or other appropriate means. A wetting agent also may beincluded in the slurry. The viscosity of the paste or paint used isdetermined by the amount and type of the solvent employed. If a flux ismixed with the aluminum powder, it is advantageous to employ a salt fluxwhich is capable of fluxing or cleaning the nickel base or cobalt basealloy. Hence, an aqueous solution of the salt hereinbefore described asconstituting the fiuxing or heating bath may be advantageously employedas a vehicle for the aluminum powder. Moreover, a combination of resinsor lacquers, salt fluxes and organic liquid vehicles may be mixed withaluminum powder to form the desired paste.

Vinyl resins and acrylic resins are among the lacquers or resins whichmay be used in the aluminum paste or spray. The lacquer identified asBinder Solution 13-9571, currently manufactured and sold by Pierce &Stevens, Inc., is an example of an appropriate resinous binder. Thistype of binder normally contains about 4% or 5% solids and solvent. Thepercentage of the solid resinous constituents is not as important as thevolatility of the solvent, however, since high volatility is required topermit rapid drying of the paste after it has been applied to theturbine blade. Good results are obtained when approximately 30% to 50%by volume of aluminum powder, preferably between 200 and 400 mesh, ismixed with 10% to 20% by volume of binder solution and about 30% to 50%by volume of an appropriate thinner or solvent. Acetone or otherconventional commercial thinners may be employed. It will beappreciated, of course, that the above ranges of the constituents in thepaste composition are not critical and that a very wide variation in thecomposition may be used to obtain satisfactory results.

Alternatively, the aluminum may be hot sprayed onto surfaces of thenickel base or cobalt base turbine blade, this method commonly beingreferred to as metallizing.

Whether the aluminum is applied in the form of a paste or paint or as ahot spray, proper bonding of the aluminum coating material to the nickelbase alloy or cobalt base alloy may be effected by subsequent heating,such as by immersion in the aforementioned salt bath. The paste shouldbe allowed to dry before immersing the turbine blade in the salt so asto avoid introducing volatile matter into the hot salt. This bathprovides proper fluxing of the nickel base alloy or cobalt base alloyand simultaneously melts the coating metal or keeps it in a molten stateso as to distribute the aluminum thinly and evenly over the turbineblade. The molten salt thus prevents the formation of detrimental oxideswhich might otherwise adversely affect the resultant bond at theinterface of the aluminum-nickel alloy or aluminum-cobalt alloy and thebase metal.

The aluminum or aluminum base alloy coating material should containapproximately 80% or more aluminum in order to provide nickel base andcobalt base listed in the following Table II, the composition againbeing given in percentages by weight.

turbine blades with effective high-temperature oxidation Table I1resistance. Hence, the word aluminum, when used in 5 the claims to referto the coating material, is intended to Example 1 Ex-ampkz Example 3Example 4 include not only pure aluminum and commercially pure aluminum,but also aluminum base alloys containing at gfigg g ;8 %3 813 21% leastapproxlmately 80% alumlnum. As hereinafter ex- Silicon--- 0.60 Maxplained, an alloy consisting essentially of approximately 5 818 m 155 2%iron and the balance aluminum provides excellent Nickel 1.50- 3.50900-1200 18.00 22.00 19.00-21.00 Its Molybdenum- 4.50- 6.50 2.5 3.253.50- 4. 50 resu Tungsten 6.00- 9.00 2.00- 3.00 3.50- 5.00

Turblne rotor buckets, nozzle guide vanes and stator Oolumbium 75- 3.00-4.50 blades are all forms of turbine blades which are exposed 555mg; fig555 015; to hlgh operatlng temperatures in gas turblne engines, CobaltBalance Balance 18.00-22.00 400044.00 particularly of the axial flowtype. All of these parts may be formed of high-temperaturecreep-resistant nickel base Referring more particularly to the drawings,in FIG- alloys and cobalt base alloys provided with an aluminum- URE 1is shown a turbine bucket 10 for a gas turbine of ni l r l m n mltsurface y r in c r n the axial flow type. In accordance with theinvention this with the present invention. Accordingly, the termturturbine bucket is formed of a nickel base alloy 12 pr0- bine bladesis employed herein as encompassing these vided with a surface layer 14of aluminum-nickel alloy. various types of gas turbine enginecomponents. For purposes of description the thickness of this alloy Thealuminum-nickel alley Preteeiive ayer should in layer is considerablyexaggerated in FIGURE 1, the actual every instance be extremely thin. Ingeneral, the layer of thickness being in the order of only one or twothouthis alloy Should have a thickness of from approximately sandths ofan inch, as hereinbefore explained. It usually 0.0005 inch to 0.0025inch. An aluminum-nickel lay r is unnecessary to provide thealuminum-nickel layer over 0.0012 inch to 0.0020 inch thick ispreferred, however, the fastening portion 16 of the turbine bucket. Witha layer thiekHeSS of about 00015 inch being Con- In order to fullyunderstand the beneficial results prosidered optimum. Similarthicknesses are appropriate vided by the surface layer ofaluminum-nickel, the metaliI1 the ease of the aluminum-Cobalt Protectivey lographic structure of nickel base alloys having this layer Thethickness of the outer aluminum layer initially formed should becompared with similar alloys which do not Should not be in excess ofapproximately (1004 inch, and have this layer. Thus, reference is madeto the photoit is presently preferred that this layer have a thicknessmicrograph of FIGURE 2, showing the surface of a tip less than about0.0015 inch. of a wedge specimen formed of an uncoated nickel base Thefollowing Table I contains examples of suitable alloy 18 having thepreferred composition set forth above high-temperature, creep-resistantnickel base alloys which which has been exposed to 197 hours of cyclicheating to y be satisffletefily Provided With a thin, protective sur-1800 F. This heating has resulted in oxide penetration face layer ofaluminum-nickel in accordance with the to the approximate depthindicated by the reference nupresent invention, the compositions beinglisted in permeral 20 in FIGURE 2. The oxide afiected zone does not sentby weight: have the same desirable high-temperature properties thatTable 1 Example 1 Example 2 Example 3 Example 4 Example 5 Carbon 0.15Max. 0.35-0.45 0.07 Max 0.15 0.08 Max. Manganese 1 Max. 2-3 1 Max. 1 Max0.3-1 Chromium 15.5-17.5 23. 52e.5 19.5 19 Cobalt 2 5 Max l0-15 13. 5 10Molybdcnum 18 2-4 4. 25 10 Tungsten. 5 75-5. 25 6-8 Iron 7 5Max 2Max.5Max Vanadium 0 20.6 Tit'lm'nm 2. 5 2. 5 Aluminum 1.25 0.87 Silicon 1Max. 1 Max. 0. 75 Max 0. 65 Max. Sulfur 0 03 Max Copper. Nickel BalanceBalance Balance Balance Balance However, the nickel base alloy disclosedin United the nickel base alloy initially possessed. Moreover, asso-States Patent No. 2,688,536 Webbere et al. appears to ciated with theoxide penetration is a zone 22 of a needlebe the most outstandingturbine bucket material currently like microconstituent beneath theoxide layer. This zone, available with respect to stress-ruptureproperties, creep which is located at a progressively greater depth asthe resistance, ductility and high-temperature corrosion redepth ofoxide penetration is increased, is extremely sistance when provided witha surface layer of aluminumbrittle and possesses low stress-ruptureproperties. The nickel in accordance with this invention. This alloycomformation of this microconstituent is accompanied by disprisesapproximately 0.06% to 0.25% carbon, 13% to appearance of the normalintermetallic network found in 17% chromium, 4% to 6% molybdenum, 8% to12% all as-cast components and is believed to be formed by a iron, 1.5%to 3% titanium, 1% to 4% aluminum, 0.01% combination of degeneration ofthe intermetallic network to 0.5% boron and the balance substantiallyall nickel. and precipitation from the matrix. Dissolved oxygen For someapplications the aluminum content may be inand nitrogen gases arebelieved to be a factor in the formacreased to approximately 6% and theiron content may tion of the needle-like microconstituent. be as low as0.1% or as high as 35%. The alloy usually The microstructure of asimilar nickel base alloy 24 should not contain more than 20% iron,however. Norhaving a surface layer of aluminum-nickel alloy 26 in mallymanganese and silicon not in excess of 1% each are also included in thealloy.

Examples of high-temperature cobalt base alloys which may be providedwith a thin, protective surface layer of aluminum-cobalt by the processdescribed herein are accordance with the invention is shown in FIGURE 3.When a tip of a wedge specimen having such a layer is subjected to thetype of cyclic heating hereinbefore described, the oxide penetrationinto the base metal is completely eliminated. Instead, as can be seen inFIGURE 4, the surface of a nickel base alloy 28, which has been aluminumcoated in the above-described manner, has a relatively deepaluminum-rich alloy layer 30 which protects the base metal from theoxidizing gases. The specimen shown was exposed to 200 hours cyclicheating to 1800 F. Thus, nickel base alloys having the surface layer ofaluminum-nickel not only are completely protected against theaforementioned oxide penetration but, after exposure to cyclic heating,also show no evidence of the needle-like microconstituent associatedwith oxide penetration. This desirable result may be attributed to theformation of a protective oxide film upon the surface of the nickel basealloy and resistance to the diffusion of gases into the matrix by thediffused aluminum atoms in solid solution with the nickel base metal.

This surface oxide layer appears to the naked eye to be similar to thesurface oxide layers on uncoated specimens which have been exposed tothe same high-temperature test conditions. However, microexaminationreveals that the surface oxide layer on the coated specimens does notpenetrate through the alloy layer into the nickel base metal.

Diffusion heat treatment of the turbine blades after the aluminumcoating operation may be beneficially employed to reduce the aluminumconcentration in the surface alloy layer. Such a heat treatment ishighly desirable to maintain high-temperature properties of the nickelbase turbine blades, and it does not adversely affect the surfaceprotection afforded by the aluminum coating. In general, a diffusionheat treatment at approximately 1700 F. to 2350 F. for one to six hourshas proved to be effective, while a diffusion period of three to sixhours at a temperature between 1800 F. and 2100 F. is preferred atpresent. Highly satisfactory results have been obtained by a fivehourdiffusion heat treatment at 1800 F., followed by air cooling. A onetothree-hour heat treatment at a temperature of 2000" F. to 2150 F. isalso very effective. It is desirable to subsequently vapor blast theturbine blades for inspection purposes. The photomicrograph of FIGUREshows a wedge specimen of a nickel base alloy 32 which had beensubjected to the aforementioned type of diffusion heat treatment andthereafter exposed to 200 hours of cyclic heating to 1800 F. Theresultant differences in the microstructure of this specimen,particularly the increased depth of the aluminum-rich alloy layer 34, ascompared with the thickness of the layer 30 in FIGURE 4, is readilyapparent.

Stress-rupture tests were conducted on both untreated nickel base alloytest specimens and similar specimens provided with a surface layer ofaluminum-nickel in accordance with the present invention.Microexamination was conducted on specimens sectioned from test barswhich were stress-rupture tested at 1700 F. and 10,000 pounds per squareinch. The total time at 1700 F. included a twenty-hour pre-heat periodbefore application of the load. All test bars had been investment castin the same mold, and results were obtained on both uncoated andaluminum coated bars for direct comparison of the coating uponhigh-temperature properties.

When a nickel base alloy specimen was successively dipped in moltenaluminum and the salt flux for a total period of 30 seconds with nodiffusion heat treatment being employed and subsequently tested at 1500F., an aluminum-rich alloy layer having a thickness of 0.0045 inch to0.005 inch was obtained. This alloy layer was the most brittle of thevarious types of aluminum-nickel layers formed on the specimens tested.This brittleness is due to the high aluminum concentration in the outerportion of the alloy layer produced at the low testing temperature of1500" F. As a result of the alloying action of the aluminum coating, thediameter of the test specimen was increased approximately 0.002 inch to0.006 inch. Stress-rupture properties were slightly decreased as aresult of the aluminum coating which, of course, improved thehigh-temperature oxidation resistance of the nickel base specimen. Themicrostructure u of this alloy after 200 hours of cyclic heating to 1800F. is shown in FIGURE 4.

A test bar similar to that described in the aforementioned example wasaluminum coated in the same manner and subsequently subjected to afive-hour diffusion heat treatment at 1800 F. followed by air cooling.This procedure increased the thickness of the aluminum-rich alloy layerbetween 0.005 inch and 0.0056 inch. Moreover, this alloy layer, which isshown in FIGURE 5 after the specimen was exposed to 200 hours of cyclicheating to 1800 F., was more ductile than the layer shown in thephotomicrograph of FIGURE 4. The surface of this heat treated specimenpossessed less pronounced aluminum concentration at the surface than thespecimen shown in FIGURE 4 due to the higher diffusion temperature.However, the thick aluminum-rich alloy layer was subject to somespalling. The diameter of the diffusion heat treated aluminum coatedtest bar was approximately 0.002 inch to 0.004 inch larger than thediameter of the bar prior to the application of aluminum. These combinedsteps of aluminum coating the nickel base alloy and subsequent diffusionheat treatment resulted in slightly decreasing the stress-rupture lifeof the test bar and slightly increasing its elongation under tensilestress.

When similar aluminum coated nickel base test specimens were subjectedto a diffusion heat treatment of five hours at 2000 F. followed by aircooling, the aluminumrich alloy layer increased in thickness to between0.0056 inch and 0.0061 inch on the average. This layer was fairlyductile because of the low aluminum concentration at the surface due tothe 2000" F. diffusion temperature. However, the thick layer wassusceptible to some spalling. The increase in diameter of the test barsresulting from this aluminum coating and diffusion treatment averagedapproximately 0.002 inch to 0.005 inch. These bars showed some decreasein stress-rupture life and an appreciable decrease in elongation undertensile stress.

Other nickel base specimens of the same composition were provided with athin surface layer of aluminumnickel alloy by means of a procedure whichincluded successively pickling the aluminum coated specimens in acid andfurther diffusing the aluminum into the base metal by heat treatment.Test bars processed in this manner exhibited excellent stress-ruptureproperties. For example, it was found that the physical properties ofnickel base alloy test bars were greatly improved when these bars werecoated by means of a 10-second aluminum dip followed by a 10-second saltrinse and thereafter subjected to a treatment comprising a picklingperiod of 25 minutes in a 10% hydrochloric acid solution, five hoursdiffusion at 1800 F. and air cooling. A photomicrograph of a section ofa test bar so treated is shown in FIGURE 6 after the specimen wasexposed to 200 hours of cyclic heating to 1800 F. It will be noted thatthe aluminumnickel layer 36 on the surface of the base metal 38 isrelatively thin, the alloy layer being only about 0.0017 inch to 0.0019inch thick. Of the various procedures which may be used to aluminum coatand heat treat nickel base alloys in accordance with the subjectinvention, this latter process produces the most ductile aluminum-richalloy layer. Moreover, there is no indication that this layer tends tospall. Although a thinner layer of aluminum-rich alloy is therebyprovided on the surface of nickel base alloys, this layer possesses alow aluminum concentration at the surface. This combination of a thinaluminum-rich layer and a low aluminum concentration at its surfaceprovides the treated alloy with optimum physical properties with respectto stress-rupture characteristics and high-temperature oxidationresistance. Moreover, the above-described treatment does not measurablyincrease the diameter of a test bar.

Thus it will be seen that diffusion heat treatment after the aluminumcoating step is highly desirable to maintain the high-temperatureproperties and to provide an alloy layer which is plastic at elevatedtemperatures during deformation with no loss of oxidation resistance.This heat treatment produces these desirable results by reducing thealuminum concentration at the surface alloy layer.

As explained above, on the other hand, aluminum coated nickel baseturbine blades which have not been subjected to diffusion heat treatmentare provided with an aluminum-rich alloy layer over which there is anexcess unalloyed aluminum layer. The thickness of the asdippedaluminum-rich alloy layer may be controlled by varying the length of thetotal dip period. The term total dip period is used herein as meaningthe total exposure time of the immersed article to molten aluminum andincludes the periods of immersion in both the aluminum coating bath andthe salt flux. However, examination has indicated that the as-dippedaluminum-rich alloy layer does not solely control the final diffusedlayer thickness. The excess aluminum overlay, which forms morealuminum-rich alloy in the diffusion process, has a greater influence onthe final layer thickness. Although vibration of the turbine blades inthe salt bath removes some of the excess aluminum, an appreciable amountof the excess aluminum still remains on the surface of the blades.

Accordingly, to obtain optimum results by means of a thinner difiusedlayer of aluminum-nickel, it is advantageous to remove the excessaluminum overlay before diffusion. As indicated above, this can beeffectively accomplished by pickling the as-dipped nickel base turbineblades in a dilute aqueous solution of hydrochloric acid at atemperature of about 60 F. to 90 F. for approximately 15 minutes to 45minutes. A 10% acid solution has been found to produce excellentresults. The aluminum-rich alloy layer is not materially affected bythis pickling process. Subsequent diffusion produces an alloy layer lessthan 0.002 inch thick, as compared with a layer thickness ofapproximately 0.005 inch to 0.006 inch for aluminum coating nickel basespecimens which are not pickled before diffusion. Hence it can be seenthat when the excess aluminum overlay is removed, the thickness of thefinal diffused aluminum-nickel layer is controlled by the thickness ofthe aluminum-rich alloy which is formed during dipping. Since thisthickness may be controlled by varying the total time the nickel basealloy is immersed in the aluminum and the salt bath, we have found atotal dip period of 10 to 20 seconds to be highly satisfactory. In thismanner an aluminum coated nickel base turbine blade may be producedwhich possesses excellent stress-rupture and other physical properties,as well as corrosion resistance at elevated temperatures.

The photomicrograph of FIGURE 7 shows the microstructure of astress-rupture bar formed of a nickel base alloy having the preferredcomposition hereinbefore set forth after the bar has been tested for2807 hours at a temperature of 1700 F. under a stress of 8500 pounds persquare inch. It will be seen that both the maximum oxide depth 40 andthe depth of alloy depletion and microstructure change are appreciable.The latter zone is indicated in FIGURE 7 by the total depth of zone 40and zone 42. When this stress-rupture bar is compared with thestress-rupture bar, such as the one shown in the photomicrograph ofFIGURE 8, having the same base metal composition but provided with adiffused surface layer of aluminum-nickel in accordance with theinvention, the differences in the microstructure of the two materialsare readily apparent. This stress-rupture bar likewise was tested at1700 F. under a stress of 8500 pounds per square inch, but the test wasextended for 2919 hours. At the end of this test period, as shown inFIGURE 8, the thickness of surface oxide zone 44 is negligible. Thedepth of the aluminum-nickel alloy layer is indicated at 46.

Results of the above and other extended stress-rupture tests attemperatures between 1500 F. and 1700 F. indicate that thestress-rupture life of nickel base alloys is increased approximately 40%by the provision of a thin surface layer of aluminum-nickel by means ofthe preferred procedure described above. The aluminum-nickellayereliminates oxide penetration of the base metal, and the diffusiontreatment produces beneficial aging or precipitation in itsmicrostruction. It is under these longperiod, high-temperatureconditions that oxidation plays a major part in determining the life ofa nickel base turbine bucket alloy.

The nickel base alloys shown in the photomicrographs of FIGURES 2through 8 have the same composition, i.e., the preferred compositionhereinbefore described. The photomicrograph of FIGURE 7 is at 250magnifications, while the other photomicrographs are at 500magnifications. Marbles reagent was used as the etchant for thespecimens shown in FIGURE 2, Vilellos etch was employed in the specimensof FIGURE 6, while a modified acid ferric chloride Was used as theetchant for the specimens shown in the other photomicrographs.

The graph of FIGURE 9 compares the creep elongation of the uncoatednickel base alloy, shown by the curve 43, with the creep elongation ofthe same alloy provided with the above-described surface layer ofalumimum-nickel. The curve 50 indicates the creep characteristics of thelatter material. A tensile load of 2500 pounds per square inch and atemperature of 1500 F. were employed in these tests. The increased lifeof the treated bars is reflected in a decreased creep rate, indicatingthat the strength of the alloy can be more effectively utilized by theformation of the protective layer. This is important when ductility ismeasured in terms of extremely long periods of time at elevatedtemperature. For example, an uncoated nickel base alloy specimen, whentested at a temperature of 1700 F. under a tensile load of 8500 poundsper square inch, deformed approximately 1.1% in 2807 hours, while asimilar specimen provided with the aforementioned surface layer ofaluminum-nickel showed no measurable deformation in 2919 hours.

It also has been found that the thickness of the final aluminum-nickellayer formed during diffusion treatment may be controlled to some extentby both the temperature of the aluminum dip bath and the composition ofthe aluminum coating material. The graph of FIGURE 10 indicates theeffect of temperature and the length of the total dip period on thethickness of this layer. For example, the final difiused alloy layer onsamples dipped in pure aluminum for the preferred immersion period of 10to 20 seconds increases approximately 0.00015 inch for each 10 F. risein dipping temperature from 1300 F. to 1400" F. This is shown in FIGURE10 wherein the curves 52, 54 and 56 indicate aluminum bath tem peraturesof 1300 F., 1350 F. and 1400" F., respec tively.

As indicated by the graph of FIGURE 11, if the total dipping time anddipping temperature remain constant, the presence of a small amount ofiron in the aluminum coating bath tends to reduce the thickness of thediffused aluminum-nickellayer. In this graph the curve 58 shows theapproximate thickness of this layer when commercial 28 aluminum is usedas the coating material, while the curve 60 indicates the thickness ofthe aluminum-nickel layer when the aluminum coating material containsapproximately 2% iron. When the preferred total dip time of 10 to 20seconds is employed with a coating metal bath at a temperature ofapproximately 1350 F., the thickness of the diffused alloy layer onnickel base turbine blades coated with an alloy of 2% iron and 98%aluminum, for example, will be approximately 0.0005 inch less than thethickness of the aluminum-nickel layer formed by coating with purealuminum.

While this invention has been described by means of certain specificexamples, it will be understood that the scope of the invention is notto be limited thereby except as defined in the following claims.

We claim:

1. An oxidation-resistant turbine blade formed of a metal selected fromthe class consisting of nickel base alloys and cobalt base alloys, theWorking surfaces of said blade having a thin layer of an alloy ofaluminum with the base metal of said blade, said layer having athickness of approximately 0.0005 inch to 0.0025 inch.

2. A highly oxidation-resistant turbine blade for a gas turbine engine,said blade being formed of a nickel base alloy and having at itsfluid-contacting surfaces an integral layer of a diffusedaluminum-nickel alloy of a thickness between approximately 0.0005 inchand 0.0025 inch.

3. A creep-resistant cast turbine blade characterized by outstandingoxidation resistance at elevated temperature formed from a nickel basealloy and having its gas-contacting surfaces provided with a relativelyductile integral layer of an aluminum-nickel alloy having a thickness ofapproximately 0.0012 inch to 0.002 inch.

4. A turbine blade characterized by high oxidationresistance at elevatedtemperature, said turbine blade being formed of a cobalt base alloyhaving its fluid-contacting surfaces formed of an integral layer ofdiffused aluminum-cobalt alloy having a thickness of about 0.0005 inchto 0.0025 inch.

5. A cast turbine blade formed from a metal consisting essentially ofabout 0.06% to 0.25% carbon, 13% to 17% chromium, 4% to 6% molybdenum,1% to 6% 12 aluminum, 1.5% to 3% titanium, iron not in excess of 20%,boron not in excess of 0.5% and the balance substantially all nickel,said blade having gas-contacting surfaces provided with a thin,relatively ductile, oxidation-resistant layer of aluminum-nickel alloy.

6. A high-temperature creep-resistant cast turbine blade for a gasturbine engine formed from a metal consisting essentially of about 0.06%to 0.25% carbon, 13% to 17% chromium, 4% to 6% molybdenum, 1% to 6%aluminum, 1.5% to 3% titanium, iron not in excess of 35%, boron not inexcess of 0.5% and the balance nickel, the gas-contacting surfaces ofsaid blade being provided with a relatively ductile oxidation-resistantintegral layer of aluminum-nickel alloy having a thickness ofapproximately 0.005 inch to 0.0025 inch.

References Cited in the file of this patent UNITED STATES PATENTS2,664,874 Graham Jan. 5, 1954 2,682,101 Whitfield June 29, 19542,683,305 Goetzel July 13, 1954 2,752,567 Schaefer July 3, 19562,807,435 Hewlett Sept. 24, 1957 2,837,818 Storchheim June 10, 1958FOREIGN PATENTS 449,998 Canada July 20, 1948

1. AN OXIDATION-RESISTANT TURBINE BLADE FORMED OF A METAL SELECTED FROMTHE CLASS CONSISTING OF NICKEL BASE ALLOYS AND COBALT BASE ALLOYS, THEWORKING SURFACES OF SAID BLADE HAVING A THIN LAYER OF AN ALLOY OFALUMINUM WITH THE BASE METAL OF SAID BLADE, SAID LAYER HAVING ATHICKNESS OF APPROXIMATELY 0.0005 INCH TO 0.0025 INCH.